Gemara

(Mishnah): If Reuven told Shimon 'I sell to you a Beis Kor (a field in which 30 Se'im of seed are sown, i.e. 75,000 square Amos), be it less or more', even if there is a (Beis) Rova too little or much per (Beis) Se'ah, there is no compensation. If the error is more than this, he calculates. He returns money, and If he wants, he returns land. He returns money to help Reuven. If the extra is (Beis) nine Kavim, he returns land.

He does not return just the Rova. He returns all the extra land.

104a - Question: Our Mishnah says that we help the seller;

Contradiction (Beraisa): If the amount of land was seven and a half Kavim per Kor too little or too much, the sale stands. If it was more than this, we force the seller to sell the extra, and we force the buyer to buy it.

Answer: Really, we do not force the seller to sell the extra land. The Beraisa teaches that if the value of the land declined, he can say 'if you give it me, give to me at today's lower price.'

Question (Beraisa): When the seller makes the buyer buy the extra land, it is according to the price at the time of the original sale.

Answer: That is when the price increased.

(Mishnah): If the extra is Beis nine Kavim (he returns land).

(Rav Huna): This applies even if there were nine extra Kavim in a sale of a giant valley.

(Rav Nachman): It applies only if there were seven and a half extra Kavim for each Kor (a Rova per Se'ah). When the extra is nine Kavim, since it is a proper field, Reuven cannot force Shimon to buy it.

(Beraisa): If Reuven kept a field bordering the sold field, Shimon can force Reuven to take back the extra land, even if it is less than nine Kavim.

(Mishnah): He does not return just the Rova for each Se'ah. He returns all the extra land.

Question: Just the contrary! The Chidush is that he returns the extra Rova'im!

Answer (Ravin bar Rav Nachman): The text should say 'he does not just return the extra above the Rova'im. He also returns the extra Rova'im.

Rishonim

Rif and Rosh (7:1): The Halachah follows Rav Nachman, who says (that there is pardon for an error up to) seven and a half extra Kavim for each Kor. When the extra is nine Kavim, he returns it.

Rambam (Hilchos Mechirah 28:7): If the excess was less than nine Kabim, he returns money like at the time of the sale, to help the seller. If the excess was near another field of the seller, he returns the extra land, for the seller adjoins it to his other fields, and does not lose at all. If the excess exceeds nine Kabim, we deduct a Rova for each Se'ah. If the excess after deducting the Rova'im was (at least) nine Kabim, he gives all the Rova'im with the excess, in land, like at the time of the sale.

Rambam (8): This is when it was cheap at the time of the sale, and rose in price when he returns it. If it was expensive and became cheaper, we tell the buyer 'if you want to return money, return like at the time of the sale. If you want to return land, return according to its current value.'

Magid Mishneh: It seems that the Rambam explains that nine Kabim is above all the Rova'im. It was explained above that any amount above the Rova'im is returned, like Rav Hai Gaon says. The Rambam's Chidush is that when there is less than nine Kabim above the Rova'im, even though in all there is nine Kabim, he returns money. This is astounding. Since there is an amount proper for the seller, why can't he return land? Also the law of change of price is difficult. Whenever the law is that he returns land, it does not matter whether the price changed! If the seller wants to sell it to him, he can sell it for whatever they agree upon! The Gemara discussed only when there is not an amount proper for the seller. Then, if he agrees to take land, if the price declined, its current value must equal the initial value of the excess. If the price rose, and the owner wants money, he gets based on the current value, for it is as if he sells it to him now. The Ra'avad explains like this. This requires investigation.

Poskim

Shulchan Aruch (CM 218:9): If the excess was less than nine Kabim, he returns money like at the time of the sale, to help the seller. If the excess was near another field of the seller, he returns the extra in land, for the seller adjoins it to his other fields, and does not lose at all.

Beis Yosef (DH Hayah): The Rashbam explains that when the excess is less than nine Kabim, we force the buyer to return money to help the seller, for it is not worth having such a small field. The seller decides whether or not he wants to sell the excess. The Beraisa teaches that the buyer has the upper hand, i.e. if the price declined and the seller wants to sell, he sells at the current lower price. We force the buyer to take it, for he does not lose.

SMA (33): Even though the Shulchan Aruch cites the Rambam, it seems that he refers only to what the Rashbam and Tur agree to. I.e. there are not nine Kabim even with the Rova'im. 'To help the seller' refers to giving money, not to the price. The next Sa'if is the Rambam's opinion; it refers to nine Kabim with[out] the Rova'im.

SMA (34): The seller has better rights, i.e. he opts to get back money or land. If too little land was given, the buyer need not accept a different field less than nine Kabim. Rather, he deducts from the price.

Shulchan Aruch (10): If the excess exceeds nine Kabim, we deduct a Rova for each Se'ah. If the excess after deducting the Rova'im was nine Kabim, he gives all the Rova'im with the excess, in land, like at the time of the sale.

Gra (19): When there are nine Kabim, the buyer can return land, but if it decreased in price, he must give what now equals the initial value, for it decreased in his Reshus. If he wants to return money, the seller must sell (at the initial price). We always favor the seller, even when the price rose. It is no loss to the buyer, for he returns land like he bought. This explains the Gemara, which says that we force the seller, and the Mishnah, which says that we (always) favor the seller.

Shulchan Aruch (11): This is when it was cheap at the time of the sale, and rose in price when he returns it. If it was expensive and became cheaper, we tell the buyer 'if you want to return money, return like at the time of the sale. If you want to return land, return according to its current value.'

Rema (12): The Shulchan Aruch from Saif 9 until here is from the Rambam. The Magid Mishneh said that it requires investigation. The primary opinion holds that whenever it is less than nine Kabim, the seller can opt to take money or land. If he takes money, he takes at the cheaper price, whether the price went up or down. If the seller had an adjacent field, or if the excess was nine Kabim, he returns land, including all the Rova'im.

SMA (35): All agree that less than a Rova per Se'ah is pardoned, and that if the excess is less than nine Kabim, he returns money. The argument is when there are nine Kabim. The Rashbam and Tur hold that the buyer can force the seller to accept land. The Rambam says so only when there are nine Kabim above the Rova'im, but if not, the seller decides. Even when the excess is nine Kabim without the Rova'im, if the price declined, the buyer must the initial value of the excess in money, or land that now is worth that amount. The seller says 'it declined (in value) in your Reshus.' If it increased in value, we say that it increased in the seller's Reshus, for the excess was never sold. The seller has the upper hand unless the price did not change. Then, he must accept the excess back in land. The Gemara favors the Rambam (see Drishah below), but the reasoning is unclear. Perhaps it is because the Rova'im are fitting to be pardoned (had there not been more excess), therefore, they do not join to force the seller to accept land. The Rema wrote that the Shulchan Aruch is like the Rambam from Sa'if 9 because he understood that Sa'if 9 discusses nine Kabim without the Rova'im.

Drishah (8): According to the Tur, it should have said 'if you want to take money for the land', not 'if you want to give.' He says that 'we force the seller to sell, and the buyer to buy' is when it declined in value. If the seller wants to sell at the low price, we force the buyer to buy it. What is the Chidush? We force the buyer even if the price did not change! The Mishnah says 'if there is more than this, we calculate.' Also, why did the Mishnah say that we (always) favor the seller? According to the Rambam, the seller never loses (due to a change in price). Perush ha'Rambam is a little difficult, but he was forced to say so because Rav Nachman said 'we allow seven and a half extra Kavim for each Kor. If there are nine Kavim extra (i.e. after deducting seven and a half Kavim per Kor)...'